Yeah the numbering when it comes to the movie title is over, especially when they named X3 "X-Men: The Last Stand".

And I really doubt that FOX would give the fifth film with the original cast the title of "X-Men 4". People might be confuse with that especially if they saw DOFP as X-Men 4.

Actually it was just "xmen : the last stand" X3 was never part of the official title. It was just referenced in image (x with wolvies claws) an was essentially the codename for it. Unlike "X2: Xmen united"'s title.

After X2 anything given an x and digit is just essentially fan reference and code name